1. Field of the Invention
The present invention relates generally to the construction and use of ultrasonic imaging catheters, and more particularly to imaging catheters having reference markers which permit rotational correlation of an ultrasonic cross-sectional image produced by the catheter with a fluoroscopic planar image of the catheter within a hollow body organ, particularly a blood vessel.
Arteriosclerosis, also known as atherosclerosis, is a common human ailment arising from the deposition of fatty-like substances, referred to as atheroma or plaque, on the walls of blood vessels. Such deposits occur both in peripheral blood vessels that feed the limbs of the body and coronary blood vessels that feed the heart. When deposits accumulate in localized regions of the blood vessels, blood flow is restricted and the person's health is at serious risk.
Numerous approaches for reducing and removing such vascular deposits have been proposed, including balloon angioplasty, where a balloon-tipped catheter is used to dilitate a stenosed region within the blood vessel; atherectomy, where a blade or other cutting element is used to sever and remove the stenotic material; and laser angioplasty, where laser energy is used to ablate at least a portion of the stenotic material.
In order to more effectively direct such interventional techniques, a variety of vascular imaging devices and methods have been proposed. Of particular interest to the present invention, intraluminal imaging catheters having ultrasonic transducers at their distal end have been employed to produce cross-sectional images of a stenotic region from within a blood vessel.
The ability to produce in situ cross-sectional images of a diseased blood vessel is advantageous in several respects. First, such images permit qualitative assessment of the nature of the stenotic region in order to help select the most effective treatment modality. Of particular concern to the present invention, the cross-sectional visual information may be used to evaluate the non-symmetric nature of a stenotic region so that intervention can be directed only at regions where the stenotic material occurs and not at healthy regions of the blood vessel where the interventional procedure might cause damage. Stenotic deposits often grow eccentrically so that a given cross-section of the blood vessel may be occluded over one portion of the wall while the remaining portion of the wall is free of disease. In such cases, it is important to localize potentially damaging treatment modalities, such as laser ablation and atherectomy, only at those regions of the blood vessel wall where the stenotic material is present.
Heretofore, it has been difficult to precisely correlate the orientation of a cross-sectional image produced by an ultrasonic imaging catheter with the actual spatial orientation of the image features within the blood vessel. Many imaging catheter constructions provide no information at all relative to the rotational orientation of the catheter within the blood vessel while the image is being produced. In such cases, correlation of the image with the actual orientation of the blood vessel is difficult or impossible. While other catheter constructions include a structural member which crosses the image plane of the ultrasonic imaging device (which produces an observable artifact on the ultrasonic image), it is still difficult to correlate position of the image artifact with the actual orientation of the catheter within the blood vessel.
It would therefore be desirable to provide improved ultrasonic imaging catheters and methods for their use which would facilitate correlation of an ultrasonic cross-sectional image with the physical orientation of the catheter producing such image within a blood vessel or other body organ. In this way, regions of the blood vessel requiring therapy could be precisely located and targeted for subsequent interventional treatment. It would be further desirable if the improved catheter and method permitted such correlation to be made by fluoroscopic observation of the catheter, such as that which is employed during initial catheter placement. It would be particularly desirable if the rotational orientation of the catheter could be uniquely determined by fluoroscopic observation at any time so that the catheter orientation could be correlated with a real time ultrasonic image being produced. Most preferably, such improvements should require only minor modification of proven catheter designs so that the other functions of the catheters are substantially undisturbed.
2. Description of the Relevant Art
U.S. Pat. No. 4,794,931, to Yock (assigned to the assignee of the present invention) describes an ultrasonic imaging catheter having a rotating transducer or a rotating mirror in combination with a fixed transducer. The imaging components are located within a housing which may be ultrasonically opaque. No specific features are provided for correlating the image produced by the catheter with the rotational orientation of the catheter. U.S. Pat. No. 4,821,731, describes the use of externally generated transverse magnetic fields which are used in determining rotational orientation of an imaging catheter.
Copending application Ser. No. 07/422,935, assigned to the assignee of the present invention but naming a different inventive entity, describes an ultrasonic imaging catheter having a fixed transducer and a rotating mirror. In one embodiment of the catheter, the cross-sectional image plane of the rotating mirror is free from ultrasonically opaque obstructions except for an axial tube which receives a movable guide wire which bypasses the imaging components. The axial tube will produce a rotationally-aligned image artifact on the cross-sectional image produced by the catheter.
Medi-tech, Inc., Watertown, Massachusetts, produces a catheter designated Ultrasound Imaging Catheter Catalog No. 01-118 having a rotating transducer mounted within its distal end. The catheter body circumscribing the rotating transducer is ultrasonically transparent, but provisions are made for a movable guide wire to pass through the image plane of the transducer. The guide wire will produce an image artifact when in place during an imaging procedure.